The present invention relates to projection exposure apparatus such as used for manufacturing semiconductor devices, liquid crystal displays and the like, and in particular to such projection exposure apparatus provided with a catadioptric projection optical system having a resolution of 0.1 microns in the ultraviolet wavelength region.
In the photolithographic process for manufacturing semiconductor devices and the like, projection exposure apparatuses are used to expose the image of a pattern present on a first object such as a photomask or reticle (hereinafter generically referred to as xe2x80x9creticlexe2x80x9d) through a projection optical system and onto a second object such as a wafer, glass plate or the like (hereinafter generically referred to as a xe2x80x9cwaferxe2x80x9d) coated with a layer of photoresist. As the level of integration of semiconductor and similar devices increases, the projection optical systems used in projection exposure apparatuses needs to have increased resolution. To increase resolution, it has become necessary to shorten the wavelength of the illumination light and to increase the numerical aperture (NA) of the projection optical system.
However, when the wavelength of the illumination light is shortened, the variety of glass materials of practical use becomes limited due to the absorption of light. At wavelengths below 300 nm, the only glass materials that presently can be used are, for practical purposes, synthetic quartz and fluorite. As the Abbe numbers of both of these glass materials are not sufficiently different, the correction of chromatic aberration is problematic.
In addition, since the required optical performance of the projection optical system is extremely high, all aberrations must be substantially eliminated. To achieve such performance with a dioptric optical system (i.e., an optical system comprising just refractive lens elements), numerous lenses are needed. Unfortunately, with such systems, a decrease in transmittance and an increase in manufacturing costs are unavoidable.
In contrast, a catoptric optical system (i.e., an optical system consisting entirely of reflective elements) has no chromatic aberration. Also, the contribution to the Petzval sum is in the opposite direction as that of a lens. Consequently, a so-called catadioptric optical system that combines a catoptric optical system and a dioptric optical system does not lead to an undue increase in the number of lens elements, and the various aberrations, including chromatic aberration, can be substantially eliminated.
Various techniques have been proposed that construct a projection optical system with such a catadioptric optical system. Among these, U.S. Pat. No. 4,747,678 (Japanese Patent Application Kokai No. Sho 63-163319), U.S. Pat. No. 5,052,763 (Japanese Patent Application Kokoku No. Hei 7-111512), U.S. Pat. No. 4,685,777 (Japanese Patent Application Kokoku No. Hei 5-25170) and U.S. Pat. No. 4,779,966 disclose catadioptric optical systems in which a first intermediate image is formed midway within in the optical system.
The prior art cited above necessarily employs a spherical mirror like a concave mirror to correct aberrations. Consequently, the optical path of the light that travels from the reticle toward the spherical mirror must be separated from the optical path of the reflected light returning from the spherical mirror, and the optical path of the reflected light returning from the spherical mirror must be deflected in the direction of the wafer. For this reason, one or more plane mirrors coated with a reflective film is necessary. However, the plane mirror that deflects the rays has a reflectance with respect to P-polarized light that is different from the reflectance with respect to the S-polarized light.
In particular, if the wavelength of the exposure light is shortened, the difference in reflectance with respect to the P-polarized light and the S-polarized light due to the reflective film increases because of the reduction in the film material and the like. As a result, undesirable polarized light having directionality is generated. If polarized light having directionality is used as the projection exposure light, the imaging performance unfortunately varies due to the directionality of the reticle pattern. Regular projection optical systems have a reduction magnification, with the amount of change in imaging performance being proportional to their NA, which is larger on the wafer side. In this case, the change in imaging performance on the wafer is marked, which creates a major problem in the photolithographic process.
The present invention relates to projection exposure apparatus such as used for manufacturing semiconductor devices, liquid crystal displays and the like, and in particular to such projection exposure apparatus provided with a catadioptric projection optical system having a resolution of 0.1 microns in the ultraviolet wavelength region.
The present invention takes the above shortcomings of the prior art into consideration, and has the goal of providing a projection exposure apparatus with a catadioptric projection optical system. The system has a large numerical aperture in the ultraviolet wavelength region, a projection optical system of a practical size, and a resolution of 0.1 microns, independent of the directionality of the reticle pattern. More particularly, the present invention is a projection exposure apparatus provided with an illumination optical system that illuminates a pattern formed on a reticle, and a catadioptric projection optical system having one or more spherical mirrors, a plurality of lenses and one or more plane mirrors. The apparatus forms an image of the pattern on a photosensitive surface of the object to be exposed, wherein the illumination optical system is constructed so that it illuminates the pattern by partially polarized light, but wherein the final image is substantially unpolarized.
Accordingly, a first aspect of the present invention is a projection exposure apparatus for forming an image of a pattern present on a first object (e.g. a reticle) onto a second object (e.g., a wafer). The apparatus comprises, along an optical axis, an illumination optical system capable of illuminating the reticle with partially polarized light, and a catadioptric projection optical system arranged adjacent the reticle and opposite the illumination optical system. The catadioptric projection optical system comprises one or more substantially spherical mirrors, a plurality of refractive members, and one or more plane mirrors. The plane mirrors are designed and arranged so as to allow a substantially unpolarized image of the pattern to be formed on the object, even though the reticle was illuminated with partially polarized light.
A second aspect of the invention is the apparatus as described above, further including a angle-devating prism, preferably comprising a birefringent optical member and an isotropic optical prism. The angle-deviating prism is capable of adjusting the intensity of the polarization components making up the partially polarized light of the light beam illuminating the reticle.